Waterproof Curtain Research Articles

Influence of Filter Tube of Pumping Well on Groundwater Drawdown during Deep Foundation Pit Dewatering

The partial penetrating waterproof curtain combined with pumping wells is widely applied to deep foundation pit dewatering engineering. The filter tube of the pumping well plays a critical role on the environment effect that resulted from foundation pit dewatering. This paper investigated the impact of the filter tube on the groundwater drawdown outside the pit to provide a theoretical basis for the foundation pit dewatering design. Three patterns according to the relative position of the waterproof curtain and the filter tube, which are called wall-well patterns, namely the full-closed pattern, part-closed pattern, and none-closed pattern, have been analyzed. By taking a practice engineering case in Shanghai as an example, the relationship among the proportion of the filter tube length to the dewatering aquifer thickness, the buried depth difference of the wall-well, and the groundwater drawdown difference at both sides of the waterproof curtain are discussed by numerical simulation. The full-closed pattern is the optimal wall-well pattern on the ideal condition. The suggested and optimal values of the filter tube length to the dewatering aquifer thickness are 38.7% and 58.2%. The suggested and optimal values of the buried depth difference of the wall-well are −6.41 m and −1.92 m.

Investigation on Dewatering of a Deep Shaft in Strong Permeable Sandy Pebble Strata on the Bank of the Yellow River

This paper reports the dewatering scheme of a deep excavation in sandy pebble strata. The excavation is in high permeability strata and is close to the Yellow River, making the dewatering difficult during construction. At present, few researchers have specially studied the dewatering scheme of deep excavations in strong permeable strata near the water resource. Field pumping test was conducted before the excavation activity, and the permeability coefficient of the strata was obtained by reverse analysis. According to the characteristics of the project, the dewatering scheme of “ waterproof curtain + base grouting + pumping ” was proposed. The influence of vertical waterproof curtain and base grouting on dewatering was analyzed by numerical simulation. In the construction process, the field water table and ground settlement were measured. The results show that (1) the groundwater table versus permeability coefficient curve shows three different stages and (2) the dewatering scheme of “ waterproof curtain + base grouting + pumping ” is effective for deep excavation in strong permeable strata.

Analysis of Seepage Characteristics of a Foundation Pit with Horizontal Waterproof Curtain in Highly Permeable Strata

At present, jet-grouted horizontal waterproof curtain reinforcement has become an essential method for deep foundation pit groundwater control. However, there is still a lack of an effective theoretical calculation method for horizontal waterproof curtain reinforcement, and there is little research on the seepage laws of foundation pits under different horizontal waterproof curtain conditions. Based on Darcy’s seepage theory, theoretical analysis models of deep foundation pit seepage were established considering the effect of a horizontal curtain in a highly permeable formation. Through the established models, the calculation method of the water inflow and the water pressure under the condition of a horizontal curtain was derived. Then through indoor tests, the reliability of the theoretical calculation method was verified. Furthermore, the established theoretical calculation method is used to analyze the influence of various factors on the water inflow and the water pressure, such as the ratio of hydraulic conductivity of the horizontal curtain to surrounding soil, thickness, and reinforcement position of the horizontal curtain. It is found that the hydraulic conductivity ratio has the most significant influence on the seepage characteristics of the foundation pit. Finally, the design method was applied to an example of the horizontal waterproof curtain of the foundation pit, which is located at Juyuanzhou Station in Fuzhou (China). The water inflow per unit area is 0.36 m3/d in the foundation pit, and this implies that the design method of the horizontal waterproof curtain applied for the excavation case is good and meets the requirements of design and safety.

Numerical modeling of foundation pit dewatering based on Visual Modflow

After the investigation of a foundation pit project and the preliminary determination of the precipitation design scheme, the distribution of water level drawdown and land subsidence in the whole site was simulated by Visual Modflow software. The simulation results show that by setting the pump output of the 14 confined dewatering wells in the pit varying from 70 ~ 420 m3/day, the drawdown of the foundation pit can be uniformly reduced to about 4m. Due to the waterproof curtain, the waterline distribution inside the foundation pit is relatively uniform, which proves the rationality of the setting of the pumping well. At the same time, the waterline contour around the edge of the curtain is very intensive, indicating that there is huge difference in the drawdown between inside and outside of the waterproof curtain. The curtain plays an effective role in controlling the drawdown inside and outside the pit foundation, hence, the compression deformation and settlement of the soil are also small so as to avoid the damage of the surrounding buildings due to the uneven settlement of the ground surface. Therefore, the precipitation design has achieved a good effect.

Study on the Factors Affecting the Quality of Trench Cutting Remixing Deep Wall in Qingdao Area

As a new type of waterproof curtain, several station construction of Qingdao Metro Line 1 successfully used the trench cutting remixing deep wall (TRD). It is difficult to find the factors that affect the wall quality because the method is in situ underground excavating and mixing. This paper has obtained the main factors affecting the TRD quality in Qingdao by studying construction experience and carrying out tests. Geological conditions are fundamental factors. Among them, the parameters of medium-coarse sand layer, the groundwater table, and the humus content are the main factors. The design parameters are the key to high-quality TRD. In the design parameters, the depth inserting into the bedrock, the cement soil ratio, and the mixing time are important factors. And this paper proposes the methods of improving the wall quality, which can provide a theoretical guarantee for the follow-up application of the TRD in the Qingdao area.

Impact of the Depth of Diaphragm Wall on the Groundwater Drawdown during Foundation Dewatering Considering Anisotropic Permeability of Aquifer

Foundation dewatering combined with a waterproof curtain is widely applied to ensure the safety of the foundation pit in areas with multi-aquifer–aquitard alternative strata. The buried depth of the diaphragm wall can influence the environmental effect due to dewatering obviously. This paper investigates the impact of the buried depth of the diaphragm wall on the groundwater drawdown considering the anisotropic permeability of the dewatering aquifer. Numerical simulation is conducted based on an engineering case. The ratio of penetrating depth of diaphragm wall to thickness of dewatering aquifer (RW) and the ratio of horizontal and vertical hydraulic conductivity of dewatering aquifer (RC) are varied. The relationship between approximate hydraulic gradient (Δi) and RW (or RC) can be fitted by Boltzmann curve (or logarithmic curve). Effective, suggested and control values of RW (or RC) are proposed, of which the suggested value is recommended in practical engineering. The effective, suggested and control value of RW can be calculated by logarithmical equation considering the value of RC.

QUALITY ASSURANCE OF DIAPHRAGM AND PILE WALLS BY GEOPHYSICS

Underground structures like diaphragm and pile walls are constructed to organize waterproof curtains, protect pit sides, and transfer loads from the structures. Violations of the construction technological process can lead to the formation of defects. To prevent adverse consequences, before excavation, it is necessary to control the integrity of the slurry walls using non-destructive geophysical methods. A review of geophysical slurry wall quality control methods based on the excitation and registration of physical fields through access tubes installed in the reinforcement cage, in wells drilled near the structure or on the surface of the structure, is presented. The main capabilities and limitations of the methods are given. Cross-hole ultrasonic logging was used to study the section of the slurry wall during the construction of the Moscow Metro station. The results of parallel soundings made it possible to identify anomalous zones, interpreted as defects. The geometric dimensions of one of the major defects were refined by ultrasonic tomography. For the first time in Russian testing practice, thermal integrity profiling was applied to study the diaphragm wall at the base of a residential building. The results of temperature monitoring during the concrete hardening are presented. According to the method, a major flaw, excess of the design mark of the wall bottom and bulging of the structure were revealed. The results of thermal integrity profiling were verified by ultrasonic logging. The combined use of thermal and ultrasonic methods can be recommended as a reliable tool for integrity testing of diaphragm and pile walls. To carry out the measurements, the access tubes shall be included in the reinforcement cage of the structure at the design stage.

Evaluation of the effectiveness of an alternative to control groundwater inflow during a deep excavation into confined aquifers

This paper presents a case study of groundwater control for a deep excavation into confined aquifers in Fuzhou, China. Field pumping tests were first carried out prior to the excavation to investigate the groundwater conditions at the site and as well as to evaluate the feasibility of previous designed dewatering systems (34 m diaphragm walls together with pumping wells). The results showed that under the condition of previous dewatering systems it would have been extremely difficult to safely lower the water level inside the excavation. A co-working scenario of partially penetrating curtains and horizontal waterproof curtain with jet grouting was then proposed to control groundwater inflows. Additional water-tightness assessment tests (WAT) were then performed that allowed to confirm the efficiency of the proposed method. From the feedbacks of WAT, this proposed scenario not only successfully lowered the water lever inside the excavation below the excavation bottom (maximum drawdown up to 14.9 m), but also it minimized the drawdown outside the excavation (less than 0.2 m). Furthermore, an approach was proposed to approximately estimate the hydraulic conductivity of the jet-grouting at the real site from the result of WAT. The hydraulic conductivity of jet-grouted soil is three orders of magnitude lower than that of the original sediments.

Calculation of groundwater head distribution with a close barrier during excavation dewatering in confined aquifer

When pumping is conducted in confined aquifer inside excavation pit (waterproof curtain), the direction of the groundwater seepage outside the excavation changes from horizontal to vertical owing to the existence of the curtain barrier. There is no analytical calculation method for the groundwater head distribution induced by dewatering inside excavation. This paper first analyses the mechanism of the blocking effects from a close barrier in confined aquifer. Then, a simple equation based on analytical solution is proposed to calculate groundwater heads inside and outside of the excavation pit with waterproof curtain (hereafter refer to close barrier) in a confined aquifer. The distribution of groundwater head is derived according to two conditions: (i) pumping with a constant water head, and (ii) pumping with a constant flow rate. The proposed calculation equation is verified by both numerical simulation and experimental results. The comparisons demonstrate that the proposed model can be applied in engineering practice of excavation.

The application of land subsidence control technologies caused by deep foundation pit dewatering

Abstract. In order to analyze land subsidence caused by the dewatering of a deep foundation pit in Shanghai, numerical analysis, field testing, automatic monitoring and other technologies are applied to research land subsidence control technologies, covering the whole process during the design, construction and operation of foundation pit dewatering systems. The key technologies are applied to a typical metro station foundation pit in Shanghai, such as the integrated design of foundation pit dewatering and a waterproof curtain, information control, groundwater artificial recharge, etc. The measured data of groundwater level, pore water pressure and land subsidence show a good effect on land subsidence prevention and control.

A three-dimensional fluid-solid coupled numerical modeling of the barrier leakage below the excavation surface due to dewatering

Waterproof curtains are used as barriers to block groundwater during deep excavations; however, in practice, barrier leakage occurs frequently. This paper establishes a three-dimensional (3D) fluid-solid-coupled finite element model to analyse the effect of barrier leakage below the excavation surface. Uniform and local barrier leakages in a confined aquifer are simulated for a constant flow rate and fixed drawdown. The simulated results show that barrier leakage causes a variation in the flow direction, drawdown of the groundwater head outside of excavation, and significant additional deformation in the longitudinal direction. The factors influencing drawdown and ground settlement induced by leakage include leakage location, leakage area, barrier penetration depth, pumping-well penetration depth, and aquifer anisotropy. In addition, the simulation results show that the drawdown-time curves under various leakage conditions have similar patterns.

Development of a trench cutting re-mixing deep wall method model test device

The trench cutting re-mixing deep wall (TRD) is a new type of underground waterproof curtain. Mixing uniformity is the key index affecting the efficiency and quality of this method. However, because of many influencing factors, existing theories cannot be used to express the relationship between various factors and mixing uniformity. By analyzing the cutting and mixing process of the TRD method, the main factors affecting the uniformity of the mixing were obtained. A model test device was designed and manufactured, based on Buckingham’s pi theorem. The validity of the model test device was verified through a comparative analysis of model and field test results. The model test device was demonstrated to be able to simulate the mixing process of the TRD method. The results provide guidance for promotion and better application of the TRD method.

Research on Quality Evaluation Method of TRD Cement-soil Continuous Wall Based on Analytic Hierarchy Process

In order to accurately evaluate the wall quality of cement-soil mixing wall by TRD method and reduce the risk of foundation pit excavation, this paper proposes a comprehensive evaluation method of wall quality. Based on strength analysis, impermeability analysis, borehole TV analysis and electromagnetic borehole radar analysis, the main influencing factors of TRD cement soil wall quality are analyzed and determined. Using the analytic hierarchy process, the hierarchical structure model of the wall quality evaluation index system is established and the evaluation factors are quantified. The fuzzy comprehensive evaluation method is used to evaluate the index factors. By analyzing the evaluation results, the evaluation grade standards of excellent, good, medium and poor reflecting the wall quality are established. The evaluation system and grade standard are used to evaluate the wall quality of waterproof curtain in a station foundation pit of Qingdao metro. The actual excavation of the foundation pit is basically consistent with the evaluation results. It has preliminarily realized a change from qualitative and empirical to scientific and half quantitative. The research results can be promoted and applied to the similar TRD cement soil mixing wall project in the future.

Application of CSM Waterproof Curtain in Foundation Pit Engineering of Deep Saturated Sand Layer

The deep foundation pit enginerring of Suning Square project in Nanchang, Jiangxi Province adopts SC50 type CSM cement mixing pile machine construction equal thick soil cement mixing wall as the anti-seepage reinforcement curtain. In order to verify the applicability of the CSM method in the deep saturated sand layer in Nanchang area, three non position test wall tests were carried out on site. The test results show that the strength, permeability and impact on the surrounding environment meet the design requirements and achieve the expected prevention. The effect of seepage reinforcement indicates that the CSM construction technology and construction parameters used in the field test are effective and applicable, and can be used as the anti-seepage reinforcement measures for the deep foundation pit engineering, and provide scientific basis and reference for the promotion and application of similar projects.

Analyses of leakage effect of waterproof curtain during excavation dewatering

The waterproof curtain that is used to block groundwater, leaks occasionally during deep excavation and can have a harmful impact on the surroundings. This study adopts a three-dimensional (3D) fluid-solid coupled finite element model (FEM) to analyze the impact due to a leakage in the waterproof curtain during excavation dewatering. In the model, both a user-defined one-dimensional leakage element and a solid leakage element are incorporated to simulate the leakage. The leakage of the curtain in the soil is analysed by monitoring the variation in hydraulic conductivity. The simulated results demonstrate that the leaking rate, leaking volume, groundwater drawdown, and ground settlement are closely related to the hydraulic conductivity of the leakage point. The results also show that the groundwater head outside the pit declines quickly when the leakage occurs, but soil consolidation proceeds slowly. In addition, the characteristics of the soil that is adjacent to the leakage point also have a tangible effect on the groundwater head and ground settlement. The hydraulic conductivity and deformation modulus of the soil has a significant effect on the ground settlement, while the influence of the anisotropy of the soil is relatively low.

高压旋喷止水帷幕复合固化剂性能分析

高压旋喷止水帷幕复合固化剂性能分析

Evaluation of optimized depth of waterproof curtain to mitigate negative impacts during dewatering

Dewatering using a combination of waterproof curtain and pumping wells is commonly adopted to ensure the stability of foundation pits in multi-aquifer strata. However, dewatering is likely to result in environmental problems. This paper investigates the impact on surroundings owing to the interaction between the waterproof curtain and pumping wells by numerical simulation. The depth of the diaphragm wall penetrating the dewatering aquifer (D) and the filter length of pumping wells (L) are varied in the simulation. The relationship between approximate hydraulic gradient (Δi) or ground settlement outside the foundation pit (S) with curtain depth ratio RD (D over the thickness of the dewatering confined aquifer Ha) or filter length ratio RL (L/Ha) can be divided into three change parts, namely initial gradual, middle sharp and final gentle part. Three values of RD and RL, namely effective, suggested and control value, are proposed. RD and RL should vary between the effective and control value. The suggested value of RD and RL is recommended in actual engineering by comprehensive consideration. The effective, suggested and control value of RD can be calculated by a linear equation according to the value of RL.

Experimental investigation on the blocking of groundwater seepage from a waterproof curtain during pumped dewatering in an excavation

The interaction between a waterproof curtain and withdrawal wells (or simply, the wall-well effect) is a key factor in controlling the environmental effects that occur during the dewatering of foundation pits. A series of laboratory investigations was conducted to study the changes in groundwater level during dewatering. The influence of the buried depth of the waterproof curtain placed in a dewatering confined aquifer (D), and the filter length of the pumping well (L), is investigated. Based on the laboratory test results, numerical simulations are used to analyse the ground settlement. The relationship between the approximate hydraulic gradients on both sides of the wall (Δi), or the ground settlement outside the excavation (S), using ratio RD (D divided by the thickness of the confined aquifer) or ratio RL (L divided by the thickness of the confined aquifer), can be simulated using a Boltzmann curve. The value of Δi increases while S decreases with an increase in RD, whereas Δi decreases while S increases with an increase in RL. The value of the contraflexure point of the Boltzmann curve is regarded as the effective value of RD or RL. Based on a comprehensive consideration, RD is suggested to be equal to or larger than the effective value, and RL is suggested to be equal to or lesser than the effective value.

Quality Evaluation and Applicability Analysis of TRD Method in Sand Stratum of Subway Station

Trench re-mixing and cutting deep method (TRD) construction is a rising method for building waterproof curtains in deep foundation pits, providing the advantages of large construction depth, widespread applicability to soil layers, and high quality of the cement-soil wall. Against the background of the Miaotou subway station foundation pit project through the quaternary system watery sand layer containing cohesive soil in Qingdao, the applicability of cement-soil wall constructed by the TRD method in Qingdao’s soft soil was researched. A comprehensive testing method which can evaluate the quality of the cement-soil wall was proposed. A series of field detection and laboratory tests that include borehole water pressure tests, borehole television imaging, electromagnetic wave borehole radar, and unconfined compressive strength test of the drilling core was completed. The test results show that the cement-soil wall had good integrity and continuity without weak tectonic zones. The average strength of the cement-soil wall ranged from 0.94 to 1.21 MPa, and the strength of the cement-soil wall at different depths existed large discreteness. However, it still met the design requirements of no less than 0.8 MPa. Test results showed that the permeability of the clay layer decreased from 10−5 to 10−7 cm/s and the permeability of the sand layer decreased from 10−2 to 10−7 cm/s. Both strength and permeability of the wall satisfied the design requirements for a watertight screen. The research results may provide reference for pit waterproof curtain engineering in similar sand stratum.

Effects of Soil Properties on the Performance of TRD Cut‐Off Wall

The trench cutting remixing deep wall (TRD) method is a new cement soil cut‐off wall construction technique, which has been widely used in cofferdam, embankment dam, and underground waterproof curtain structures. The chain cutter of TRD moves horizontally to cut and stir different soil layers by up and down to form a cement‐mixed soil diaphragm wall with continuous and uniform thickness, the mechanics and permeability of which are obviously different from those of cement soil with a horizontally mixed single soil layer in traditional deep mixing pile (DMP). In this paper, five sets of onsite walling tests with different cement ratios were carried out to analyse the unconfined compression and permeability of undisturbed cement‐mixed soil in TRD. The difference of both unconfined compressive and permeability coefficients in between TRD and DMP was analysed to discuss the stirring performance of TRD. Then, the microscopic mechanism of soil properties affecting the performance of cement‐mixed soil has been studied by scanning electron microscopy (SEM) in the unconfined compression strength test and permeability coefficient test of cement soil with different mixed soil properties. The test results show that the unconfined compressive strength of cement‐mixed soil is closely correlated to its gradation. As the mixed soil gradation curve approaches to the Fuller curve, the unconfined compressive strength would gradually increase. In mesoscopic, cement fine sand has a large pore structure, and the average pore area is 2.46 times as cement clay. The permeability coefficient of cement‐mixed soil is controlled by the proportion of fine sand content with high permeability.